The present invention pertains generally to opto-electronics, and more particularly, to a method and circuit that controls the effective light level in a photodetector array by use of an electronic shutter.
Photo-detector arrays are used in many opto-electronic devices that involve imaging applications. One common application is known as an optical mouse, which is used in conjunction with a computer display to assist in navigation and capturing user input.
The typical optical mouse includes an illumination LED and a lens. An electronic shutter is controlled electronically to allow or disallow light from entering the photodetector array. A shutter control circuit monitors the light intensity across the photodetector array to determine and control the amount of allowed exposure time seen by the array. FIG. 1 illustrates a typical imaging application which employs an opto-electronic device 10. As illustrated, device 10 includes a photodetector array 2 positioned in the image plane of an optic lens 4. An illumination source 6 illuminates an image 12 positioned in the focal range of the image plane of optic lens 4. Light from the image passes through lens 4 and is sensed by the individual photodetectors in the photodetector array 2. Light reaching the photodetectors is controlled by the length of time a shutter 8 is open. In opto-electronic devices, the photodetector exposure is controlled electronically. For example, device 10 includes a shutter control circuit 14 which processes the light intensity level sensed by the individual photodetectors in the photodetector array 2 to determine an appropriate length of time in which the shutter should be open in order to produce the appropriate amount of exposure, and controls the opening and closing of the shutter in accordance with the determined amount of shutter open time.
The advantages of automatic shutter control are somewhat self-evident, and include the fact that no user intervention is required to set an appropriate shutter speed, and that the calculation and setting of the shutter speed electronically is much faster than a user could do the same, to name only a few. In many applications, the automatic adjustment of shutter speed is critical in facilitating high-rate collection of multiple image frames, as is necessary in the optical mouse example. Thus, it is quite clear that automated electronic shutter speed control is desirable, and quite often necessary, in many of today""s imaging applications.
However, the technique used in prior art shutter control circuits is problematic in certain situations. Prior art shutter control circuits typically only attempt to keep the peak light level within a predetermined optimal range so as to obtain the best signal-to-noise ratio and best image quality. Because the shutter is controlled based only upon the peak light level, certain types of images are often not properly detected. For example, when attempting to optically navigate on a shiny black surface (e.g. Lycra or Spandex), an example of which is illustrated in FIG. 2, the photodetector array typically senses only a small few very bright pixels 20 where the illumination source is directly reflected into the photodetector array. Bright pixels are darkened in this illustration. The individual photodetectors in the photodetector array sense the light level on their corresponding pixel of the true image and generate a voltage level in proportion to the sensed light level. The output from the individual photodetectors is converted into digital format and concatenated into a stream 25 of pixel intensity data. A peak detector (not shown) processes the pixel intensity stream 25 frame by frame, detects whether any of the pixels are above a maximum allowed intensity level VHIGH, and decreases the amount of time the shutter is open if any of the pixels is above the threshold VHIGH. Accordingly, in the image example of FIG. 2, the pixel intensity stream 25 for a single frame, as illustrated in FIG. 3A will remain very close to the lower threshold VMIN except for the few very bright pixels 20, which will peak near the maximum allowed intensity level threshold VHIGH. Because the shutter 8 in prior art shutter control circuits 14 is controlled based only upon the peak light intensity level, the exposure time will be decreased based on these few bright pixels 20 rather than the intensity level of the image as a whole. Accordingly, the shutter open time, as illustrated in FIG. 4, will be decreased.
In an ideal photodetector array, the full range of intensity levels for the entire image will fall within the lower and upper threshold limits VMIN and VHIGH. Accordingly, if the peak detector detects a pixel intensity level outside this range, the exposure time must be adjusted to place the image completely within the range. Thus, when the most part of the information generally occupies intensity levels far lower than the maximum intensity level, but a few very bright pixels are detected due to the reflective surface of the image, because the exposure time is adjusted to scale the entire image including the very bright pixels within the full intensity range of the image processing circuitry as illustrated in FIG. 5A, the remainder of the image will be seen as mostly black. Accordingly, the optical device 10 is essentially xe2x80x9cblindedxe2x80x9d, and if the remainder of the image contains information, the information will be very difficult, if not impossible, to detect.
Accordingly, a need exists for an improved method and apparatus for adjusting the amount of exposure time via the amount of shutter open time.
The present invention is a novel method and circuit that controls the effective light level in a photodetector array (PA) by use of an electronic shutter. The circuit uses average and peak light level data to determine the electronic shutter time necessary for controlling the light level of the PA.
In accordance with the invention, the average and peak light levels are monitored using an illumination source and lens. If the average light level is below a first threshold, the shutter open time is increased. Otherwise, if the peak light level is below a second threshold, the shutter open time is increased; if the peak light level is above a third threshold, the shutter open time is decreased; otherwise, the shutter open time is not adjusted. In other words, using the shutter open time to control the light level, the control circuit attempts to keep the average light level above the first threshold. If the average light level is above the first threshold, then the peak value is maintained between the second and third thresholds. Accordingly, using the average light level in the feedback loop, very bright pixels are ignored, and the shutter is set open long enough to allow the remainder of the image to be visible to the photodetector array, resulting in more texture to be used with correlation navigation.
In one embodiment, a shutter control circuit receives a pixel stream containing serialized pixel intensity data that was detected by a photodetector array and converted to digital form. The serialized pixel intensity data for one entire frame is input to an average accumulator, which calculates the average intensity over the entire image frame. The average intensity is compared to a minimum average threshold level, generating an average threshold signal which indicates whether the average frame intensity is below the minimum average threshold level. If so, control logic increases the pulse length of the enable shutter line which controls the amount of shutter open time. Simultaneously, the serialized pixel intensity data is also input to a peak detector, which determines whether any of the pixels were above a maximum allowed peak intensity level over the entire image frame. The output of the peak detector is input to a comparator circuit which then compares the peak intensity to a minimum peak threshold and a maximum peak threshold. If the peak intensity is below the minimum peak threshold, control logic increases the pulse length of the enable shutter line which controls the amount of shutter open time. If the peak intensity is above the maximum peak threshold, control logic decreases the pulse length of the shutter enable control line. When the peak intensity is neither below the minimum peak intensity nor above the maximum peak intensity, the pulse length of the shutter enable control line remains the same.